The present invention relates to an apparatus for measuring an adhesion force of a thin film to a substrate.
Thanks to the progress of film forming technology, many kinds of thin films are extensively used in a variety of fields as magnetic materials, electronic materials and corrosion-resistant materials. A problem with such a thin film device is that the thin film is apt to come off its associated substrate or another thin film in various situations, e.g., when the film is relatively thick, when the film is constituted multiple layers each being made of a different material, when the surface of the film suffers from some contamination during the course of production, and when the use of materials which do not match each other is unavoidable for the substrate and thin film. The separation of the substrate and the thin film during the course of manufacturing immediately leads to a decrease in yield and an increase in cost. Another problem with the thin film device is the separation of the thin film due to heat and corrosion which depend upon the ambient conditions. Separation ascribable to temperature variation and the corrosion often effect the resistivity to environment and long-term reliability of the thin film.
For the improvement in quality control and reliability, therefore, there is a keen demand for a method capable of measuring an adhesion force of the thin film with high accuracy and thereby promoting quantitative analysis of separation tendency of the thin film.
Some elaborated methods have heretofore been proposed for the measurement of the adhesion forces of the thin film to the substrate. All the conventional methods are of the sort measuring a force or energy needed to remove a thin film from the substrate and may generally be classified into five types, i.e., (1) an adhesion type method, (2) a scratch type method, (3) a centrifugal force type method, (4) a ultrasonic wave type method, and (5) an electromagnetic tension type method. Of these five different methods, the scratch type method is predominant presently because it is superior in the quantitative nature and reproducibility of data.
The scratch type method is such that a hard stylus is pressed against a thin film under a certain load and then moved to scratch the thin film from a substrate, as described in the article "Adhesion of TiC and Ti(C,N) coatings on steel" by P. A. Steinmann et al. (J. Vacuum Science Technology, A3 (6), pages 2394-2400, issued 1985). In this method, the greatest shearing force acts in edge portions of an indent which is formed by the stylus and the separation proceeds at the edge portion. The stylus has a tip made by diamond, sapphire or similar hard material and provided with an appropriate radius of curvature. The adhesion force acting between the thin film and the substrate is defined as a load which forms the scratch on the surface of the thin film or a load which sharply increases the coefficient of friction.
The scratch type measuring apparatus further includes an acoustic emission (AE) sensor for measuring the adhesion force. The AE sensor measures the adhesion force by causing the diamond stylus to scratch the surface of the thin film at a constant speed. The load acting on the thin film is sequentially increased from 1 to 200N at a predetermined rate (usually 100N/min) during one scratching operation. As the scratching operation proceeds under the increasing load, the AE sensor mounted on an indenter holder produces an AE signal which is associated with destruction in the thin film or at the boundary between the thin film and the substrate. A particular load which causes the AE signal to sharply increase (hereinafter referred to as a critical load) is defined as the adhesion force of the thin film.
The conventional apparatus for the measurement of the adhesion force have various problems left unsolved. Since the critical load measured by the scratch type method is complicatedly dependent upon the hardness of the substrate and the thickness of the thin film, maintaining consistent substrate hardness and film thickness is required to measure the critical loads. Another requirement for the measurement of the critical loads is that the surfaces of the substrates and the thin films be individually finished in the same condition because the surface roughness of the substrates and the thin films also has influence on the critical load.
Further, the conventional apparatus can measure the thin film thickness up to micron range. However, the measurement in the micron thickness range is unsatisfactory for the current thin film forming technology which has reached a submicron thickness range. In the scratch type measurement to a ductile material, a swell is formed by the stylus so as to increase the coefficient of friction and cause the acoustic emissions and, therefore, it is difficult to evaluate the adhesion forces of highly ductile thin films by the scratch type measurement. For the reasons described above, the applicable range of the scratch type measurement is limited to oxide films, nitride films and carbide films which are relatively hard and lie in the micron thickness range.